186 , STATE HORTICULTURAL SOCIETY. 



Even when the entire top was removed, as by the tornado in G 17, the 

 disease quickly reappeared. 



The inference that the disease was really cut out and that all these cases 

 are reinfections is inadmissible, because nothing of a similar nature occur- 

 red elsewhere in the orchards. To establish this we have only to trace the 

 history of these trees for one year from the date of the excisions, and to 

 compare that with the history for a similar period of other trees in each 

 orchard, i. e., those which were healthy when the excisions were made. 



In Mr. Geeen's orchard 100 per cent, of the excised trees showed fresh 

 symptoms of the disease in 1888, but only about twelve per cent, of the 

 others became affected; i. e., the new cases in 1888 amounted to only 

 twelve per cent, of the whole number of healthy trees. In Mr. Haepee's 

 orchard ninety-five per cent, of the excised trees showed fresh symptoms 

 of the disease in 1889, but only about two per cent, of the others became 

 affected. Most of the excisions were made in these two orchards, and no 

 record of cases in the other orchards was kept for comparison. 



In a few instances I have known fifty per cent, of the trees of an orchard 

 to become affected with yellows in one year, but never one hundred per 

 cent., and very rarely more than fifteen to twenty-five per cent., even in 

 the worst affected districts. It would seem, therefore, that the disease* 

 persisted in these trees, i. e., that the symptoms subsequent to the excis- 

 ions were not the result of reinfection. 



It remains to ask whether the excisions exerted any retarding influence. 

 One year from the date of the excisions the trees in Mr. Haepee's orchard 

 were freer from symptoms of yellows than had been those in Mr. Geeene's 

 orchard (Experiment B), and the same relative difference was apparent at 

 the end of the second year. It might not be proper, however, to draw the 

 inference that the development of the disease was slower on account of the 

 excisions. There is some doubt as to whether the excisions retarded the 

 progress of the disease, for the following reasons: (1) The trees manifested 

 only very slight symptoms to begin with, i. e., much slighter than the trees 

 in Mr. Geeene's orchard; (2) they bore no fruit in 1889 or 1890, whereas 

 the trees in Mr. Geeene's orchard experienced the strain of a large crop in 

 1888, and of a partial one in 1889; (3) the natural progress of this disease 

 is sometimes very slow, requiring a third season for the development of 

 the symptoms in all parts of the tree. It is therefore not impossible that 

 the disease might have progressed in these particular trees with the same 

 slowness, had no limbs been removed. The most that can be said is that 

 the severest excisions, e. g., those in Mr. Haepee's orchard, appeared to 

 exert a retarding influence on the progress of the disease. 



Of course, the results obtained do not preclude the possibility of cutting 

 out the disease in some cases. The evidence to the contrary is, however, 

 reasonably conclusive, and sufficient for all practical purposes. The 

 experiments also throw considerable light upon the nature of the disease 

 but do not settle the question as to whether the disease is latent in the 

 whole tree when symptoms appear in any part of it. This will require 

 additional experiments of another kind. 



IV. — obseevations and expeeiments beaeing upon immunity. 



(1) The question of immunity is a very important one. If we could 

 somewhere find peach trees hardy enough to resist this disease, a great 

 practical problem would be solved. It has been said that seedlings are 

 much less subject to yellows than budded trees. In the localities where 



